Chapter 25 Beyond Our Solar System

Section 1 Properties of Stars

Key Concepts What can we learn by studying star properties? How does distance affect parallax? What factors determine a star’s apparent magnitude? What relationship is shown on a Hertzsprung-Russell diagram?

Vocabulary constellation binary star light-year apparent magnitude absolute magnitude main-sequence star red giant supergiant Cepheid variable nova nebulae

The star Proxima Centauri is about 100 million times farther away from Earth than the moon. Yet, besides the sun,it is the closest star to Earth. The universe is incomprehensibly large. What is the nature of this vast universe? Dostars move, or do they remain in one place? Does the universe extend infinitely in all directions, or does it haveboundaries? This chapter will answer these questions by examining the universe and the most numerous objects inthe night sky—the stars.As early as 5000 years ago, people became fascinated with the star-studded skies and began to name the patternsthey saw. These patterns of stars, called constellations, were named in honor of mythological characters or greatheroes, such as Orion, shown in Figure 1.

Figure 1 Orion The constellation Orion was named for a hunter.

Although the stars that make up a constellation all appear to be the same distance from Earth, some are many timesfarther away than others. So, the stars in a particular constellation are not associated with one another in anyphysical way.Today 88 constellations are recognized. They are used to divide the sky into units, just as state boundaries dividethe United States. Every star in the sky is in, but is not necessarily part of, one of these constellations. Therefore,constellations can be used as a “map” of the night sky.

Characteristics of StarsA great deal is known about the universe beyond our solar system. This knowledge hinges on the fact that stars, andeven gases in the “empty” space between stars, radiate energy in all directions into space. The key to understandingthe universe is to collect this radiation and unravel the secrets it holds. Astronomers have devised many ways to dojust that. We will begin by examining some properties of stars, such as color, temperature, and mass.

Star Color and TemperatureStudy the stars in Figure 2 and note their color. Color is a clue to a star’s temperature. Very hot stars with surfacetemperatures above 30,000 K emit most of their energy in the form of short-wavelength light and therefore appearblue. Red stars are much cooler, and most of their energy is emitted as longer-wavelength red light. Stars withtemperatures between 5000 and 6000 K appear yellow, like the sun.

Figure 2 Stars of Orion This time-lapse photograph shows stars as streaks across the night sky as Earth rotates. Thestreaks clearly show different star colors.

Binary Stars and Stellar MassIn the early nineteenth century, astronomers discovered that many stars orbit each other. These pairs of stars, pulledtoward each other by gravity, are called binary stars. More than 50 percent of the stars in the universe may occur inpairs or multiples.Binary stars are used to determine the star property most difficult to calculate—its mass. The mass of a body can becalculated if it is attached by gravity to a partner. This is the case for any binary star system. As shown in Figure 3,binary stars orbit each other around a common point called the center of mass. For stars of equal mass, the center ofmass lies exactly halfway between them. If one star is more massive than its partner, their common center will becloser to the more massive one. If the sizes of their orbits are known, the stars’ masses can be determined.

Figure 3 Common Center of Mass A For stars of equal mass, the center of mass lies in the middle. B A star twice asmassive as its partner is twice as close to the center of mass. It therefore has a smaller orbit than its less massivepartner.

Measuring Distances to StarsAlthough measuring the distance to a star is very difficult, astronomers have developed some methods ofdetermining stellar distances.

ParallaxThe most basic way to measure star distance is parallax. Parallax is the slight shifting in the apparent position of anearby star due to the orbital motion of Earth. Parallax is determined by photographing a nearby star against thebackground of distant stars. Then, six months later, when Earth has moved halfway around its orbit, a secondphotograph is taken. When these photographs are compared, the position of the nearby star appears to have shiftedwith respect to the background stars. Figure 4 shows this shift and the resulting parallax angle.

Figure 4 Parallax The parallax angle shown here is exaggerated to illustrate the principle. Because the distances toeven the nearest stars are huge, astronomers work with very small angles. Relating Cause And Effect What causedthe star to appear to shift?

The nearest stars have the largest parallax angles, while those of distant stars are too small to measure. In fact, allparallax angles are very small. The parallax angle to the nearest star (besides the sun), Proxima Centauri, is lessthan 1 second of arc, which equals 1/3600 of a degree. To put this in perspective, fully extend your arm and raiseyour little finger. Your finger is roughly 1 degree wide. Now imagine tracking a movement that is only 1/3600 aswide as your finger.In principle, the method used to measure stellar distances may seem simple. But in practice, measurements aregreatly complicated because of the tiny angles involved and because the sun, as well as the star being measured,also move through space. Even with today’s technology, parallax angles for only a few thousand of the nearest starsare known with certainty.

Light-YearDistances to stars are so large that units such as kilometers or astronomical units are often too hard to use. A betterunit to express stellar distance is the light-year , which is the distance light travels in one year—about 9.5 × 1012or9.5 trillion kilometers. Proxima Centauri is about 4.3 light-years away from the sun.

Stellar BrightnessThe measure of a star’s brightness is its magnitude. The stars in the night sky have an assortment of sizes,temperatures, and distances, so their brightnesses vary widely.

Apparent MagnitudeSome stars may appear dimmer than others only because they are farther away. A star’s brightness as it appearsfrom Earth is called its apparent magnitude. Three factors control the apparent brightness of a star as seen fromEarth: how big it is, how hot it is, and how far away it is.Astronomers use numbers to rank apparent magnitude. The larger the number is, the dimmer the star. Just as we cancompare the brightness of a 50-watt bulb to that of a 100-watt bulb, we can compare the brightness of stars havingdifferent magnitudes. A first-magnitude star is about 100 times brighter than a sixth-magnitude star. Therefore, twostars that differ by 5 magnitudes have a ratio in brightness of 100 to 1. It follows, then, that the brightness ratio of

two stars differing by only one magnitude is about 2.5. A star of the first magnitude is about 2.5 times brighter thana star of the second magnitude.

Absolute MagnitudeAstronomers are also interested in how bright a star actually is, or its absolute magnitude. Two stars of the sameabsolute magnitude usually do not have the same apparent magnitude because one may be much farther from usthan the other. The one that is farther away will appear dimmer. To compare their absolute brightness, astronomersdetermine what magnitude the stars would have if they were at a standard distance of about 32.6 light-years. Forexample, the sun, which has an apparent magnitude of −26.7, would, if located at a distance of 32.6 light-years, have an absolute magnitude of about 5. Stars with absolute magnitude values lower than 5 are actually brighter thanthe sun. Because of their distance, however, they appear much dimmer. Table 1 lists the absolute and apparentmagnitudes of some stars as well as their distances from Earth.

Hertzsprung-Russell DiagramEarly in the twentieth century, Einar Hertzsprung and Henry Russell independently developed a graph used to studystars. It is now called a Hertzsprung-Russell diagram (H-R diagram). A Hertzsprung-Russell diagram shows therelationship between the absolute magnitude and temperature of stars. By studying H-R diagrams, we learn a greatdeal about the sizes, colors, and temperatures of stars.In the H-R diagram shown in Figure 5, notice that the stars are not uniformly distributed. About 90 percent aremain-sequence stars that fall along a band that runs from the upper-left corner to the lower-right corner of thediagram. As you can see, the hottest main-sequence stars are the brightest, and the coolest main-sequence stars arethe dimmest.The brightness of the main-sequence stars is also related to their mass. The hottest blue stars are about 50 timesmore massive than the sun, while the coolest red stars are only 1/10 as massive. Therefore, on the H-R diagram, themain-sequence stars appear in decreasing order, from hotter, more massive blue stars to cooler, less massive redstars.Above and to the right of the main sequence in the H-R diagram lies a group of very bright stars called red giants.The size of these giants can be estimated by comparing them with stars of known size that have the same surfacetemperature. Objects with equal surface temperatures radiate the same amount of energy per unit area. Therefore,any difference in the brightness of two stars having the same surface temperature is due to their relative sizes. Somestars are so large that they are called supergiants. Betelgeuse, a bright red supergiant in the constellation Orion, hasa radius about 800 times that of the sun.

Figure 5 Hertzsprung-Russell Diagram In this idealized chart, stars are plotted according to temperature andabsolute magnitude.

Stars in the lower-central part of the H-R diagram are much fainter than main-sequence stars of the sametemperature. Some probably are no bigger than Earth. This group is called white dwarfs, although not all are white.Soon after the first H-R diagrams were developed, astronomers realized their importance in interpreting stellarevolution. Just as with living things, a star is born, ages, and dies. After considering some variable stars and thenature of interstellar matter, we’ll return to the topic of stellar evolution.

Variable StarsStars may fluctuate in brightness. Some stars, called Cepheid variables, get brighter and fainter in a regular pattern.The interval between two successive occurrences of maximum brightness is called a light period. In general, thelonger the light period of a Cepheid, the greater its absolute magnitude is. Once the absolute magnitude is known, itcan be compared to the apparent magnitude of the Cepheid. Measuring Cepheid variable periods is an importantmeans of determining distances within our universe.A different type of variable is associated with a nova, or sudden brightening of a star. During a nova eruption, theouter layer of the star is ejected at high speed. A nova, shown in Figure 6, generally reaches maximum brightness ina few days, remains bright for only a few weeks, then slowly returns in a year or so to its original brightness. Only asmall amount of its mass is lost during the flare-up. Some stars have experienced more than one such event. In fact,the process probably occurs repeatedly.

Figure 6 Nova These photographs, taken two months apart, show the decrease in brightness that follows anova flare-up.

Scientists think that novas occur in binary systems consisting of an expanding red giant and a nearby hot whitedwarf. Hydrogen-rich gas from the oversized giant is transferred by gravity to the white dwarf. Eventually, theadded gas causes the dwarf to ignite explosively. Such a reaction rapidly heats and expands the outer layer of thehot dwarf to produce a nova. In a relatively short time, the white dwarf returns to its prenova state, where it remainsinactive until the next buildup occurs.

Interstellar MatterBetween existing stars is “the vacuum of space.” However, it is not a pure vacuum, for there are clouds of dust andgases known as nebulae. If this interstellar matter is close to a very hot star, it will glow and is called a brightnebula. The two main types of bright nebulae are emission nebulae and reflection nebulae.Emission nebulae consist largely of hydrogen. They absorb ultraviolet radiation emitted by a nearby hot star.Because these gases are under very low pressure, they emit this energy as visible light. This conversion ofultraviolet light to visible light is known as fluorescence. You can see this effect in fluorescent lights. Reflectionnebulae, as the name implies, merely reflect the light of nearby stars. Reflection nebulae are thought to becomposed of dense clouds of large particles called interstellar dust.Some nebulae are not close enough to a bright star to be lit up. They are called dark nebulae. Dark nebulae, such asthe one shown in Figure 7, can easily be seen as starless regions when viewing the Milky Way.

Figure 7 Dark Nebula The Horsehead Nebula is found in the constellation Orion.

Although nebulae appear very dense, they actually consist of thinly scattered matter. Because of their enormoussize, however, their total mass may be many times that of the sun. Astronomers study nebulae because stars andplanets form from this interstellar matter.

Section 2 Stellar Evolution

Key Concepts What stage marks the birth of a star? Why do all stars eventually die? What stages make up the sun’s life cycle?

Vocabulary protostar supernova white dwarf neutron star pulsar black hole

Determining how stars are born, age, and then die was difficult because the life of a star can span billions of years.However, by studying stars of different ages, astronomers have been able to piece together the evolution of a star.Imagine that an alien from outer space lands on Earth. This alien wants to study the stages of human life. Byexamining a large number of humans, the alien observes the birth of babies, the activities of children and adults,and the death of elderly people. From this information, the alien then attempts to put the stages of human

development into proper sequence. Based on the number of humans in each stage of development, the alien wouldconclude that humans spend more of their lives as adults than as children. In a similar way, astronomers havepieced together the story of stars.

Star BirthThe birthplaces of stars are dark, cool interstellar clouds, such as the one in Figure 8. These nebulae are made up ofdust and gases. In the Milky Way, nebulae consist of 92 percent hydrogen, 7 percent helium, and less than 1 percentof the remaining heavier elements. For some reason not yet fully understood, some nebulae become dense enoughto begin to contract. A shock wave from an explosion of a nearby star may trigger the contraction. Once the processbegins, gravity squeezes particles in the nebula, pulling every particle toward the center. As the nebula shrinks,gravitational energy is converted into heat energy.

Figure 8 Nebula Dark, cool clouds full of interstellar matter are the birthplace of stars.

Protostar StageThe initial contraction spans a million years or so. As time passes, the temperature of this gaseous body slowly risesuntil it is hot enough to radiate energy from its surface in the form of long-wavelength red light. This large redobject is called a protostar. A protostar is a developing star not yet hot enough to engage in nuclear fusion.During the protostar stage, gravitational contraction continues—slowly at first, then much more rapidly. Thiscollapse causes the core of the protostar to heat much more intensely than the outer layer. When the core of aprotostar has reached about 10 million K, pressure within is so great that nuclear fusion of hydrogen begins, and astar is born.Heat from hydrogen fusion causes the gases to increase their motion. This in turn causes an increase in the outwardgas pressure. At some point, this outward pressure exactly balances the inward force of gravity, as shown in Figure9. When this balance is reached, the star becomes a stable main-sequence star. Stated another way, a stable main-sequence star is balanced between two forces: gravity, which is trying to squeeze it into a smaller sphere, and gaspressure, which is trying to expand it.

Figure 9 Balanced Forces A main-sequence star is balanced between gravity, which is trying to squeeze it, and gaspressure, which is trying to expand it.

Main-Sequence StageFrom this point in the evolution of a main-sequence star until its death, the internal gas pressure struggles to offsetthe unyielding force of gravity. Typically, hydrogen fusion continues for a few billion years and provides theoutward pressure required to support the star from gravitational collapse.Different stars age at different rates. Hot, massive blue stars radiate energy at such an enormous rate that theydeplete their hydrogen fuel in only a few million years. By contrast, the least massive main-sequence stars mayremain stable for hundreds of billions of years. A yellow star, such as the sun, remains a main-sequence star forabout 10 billion years.An average star spends 90 percent of its life as a hydrogen-burning, main-sequence star. Once the hydrogen fuel inthe star’s core is depleted, it evolves rapidly and dies. However, with the exception of the least-massive red stars, astar can delay its death by fusing heavier elements and becoming a giant.

Red-Giant StageThe red-giant stage occurs because the zone of hydrogen fusion continually moves outward, leaving behind ahelium core. Eventually, all the hydrogen in the star’s core is consumed. While hydrogen fusion is still progressingin the star’s outer shell, no fusion is taking place in the core. Without a source of energy, the core no longer hasenough pressure to support itself against the inward force of gravity. As a result, the core begins to contract.As the core contracts, it grows hotter by converting gravitational energy into heat energy. Some of this energy isradiated outward, increasing hydrogen fusion in the star’s outer shell. This energy in turn heats and expands thestar’s outer layer. The result is a giant body hundreds to thousands of times its main-sequence size, as shown inFigure 10.

Figure 10 Life Cycle of a Sunlike Star A medium-mass star, similar to the sun, will evolve along the path shownhere. Interpreting Diagrams What is the first stage in the formation of the star? What is the last stage?

As the star expands, its surface cools, which explains the star’s reddish appearance. During expansion, the corecontinues to collapse and heat until it reaches 100 million K. At this temperature, it is hot enough to convert heliumto carbon. So, a red giant consumes both hydrogen and helium to produce energy.Eventually, all the usable nuclear fuel in these giants will be consumed. The sun, for example, will spend less than abillion years as a giant. More massive stars will pass through this stage even more rapidly. The force of gravity willagain control the star’s destiny as it squeezes the star into the smallest, most dense piece of matter possible.

Burnout and DeathMost of the events of stellar evolution discussed so far are well documented. What happens next is based more ontheory. We do know that all stars, regardless of their size, eventually run out of fuel and collapse due to gravity.With this in mind, let’s consider the final stages of stars of different masses.

Death of Low-Mass StarsAs shown in Figure 11A, stars less than one half the mass of the sun consume their fuel at a fairly slow rate.Consequently, these small, cool red stars may remain on the main sequence for up to 100 billion years. Because theinterior of a low-mass star never reaches high enough temperatures and pressures to fuse helium, its only energysource is hydrogen. So, low-mass stars never evolve into red giants. Instead, they remain as stable main-sequencestars until they consume their hydrogen fuel and collapse into a white dwarf, which you will learn more about later.

Figure 11 Stellar Evolution A A low-mass star uses fuel at a low rate and has a long life span. B Like a low-massstar, a medium-mass star ends as a black dwarf. C Massive stars end in huge explosions, then become either neutronstars or black holes.

Death of Medium-Mass StarsAs shown in Figure 11B, stars with masses similar to the sun evolve in essentially the same way. During their giantphase, sunlike stars fuse hydrogen and helium fuel at a fast rate. Once this fuel is exhausted, these stars alsocollapse into white dwarfs.During their collapse from red giants to white dwarfs, medium-mass stars are thought to cast off their bloated outerlayer, creating an expanding round cloud of gas. The remaining hot, central white dwarf heats the gas cloud,causing it to glow. These often beautiful, gleaming spherical clouds are called planetary nebulae. An example of aplanetary nebula is shown in Figure 12.

Figure 12 Planetary Nebula During its collapse from a red giant to a white dwarf, a medium-mass star ejects itsouter layer, forming a round cloud of gas.

Death of Massive StarsIn contrast to sunlike stars, which die gracefully, stars with masses three times that of the sun have relatively shortlife spans, as shown in Figure 11C. These stars end their lives in a brilliant explosion called a supernova. During asupernova, a star becomes millions of times brighter than its prenova stage. If one of the nearest stars to Earthproduced such an outburst, it would be brighter than the sun. Supernovae are rare. None have been observed in ourgalaxy since the invention of the telescope, although Tycho Brahe and Galileo each recorded one about 30 yearsapart. An even larger supernova was recorded in 1054 by the Chinese. Today, the remnant of this great outburst isthe Crab Nebula, shown in Figure 13.

Figure 13 Crab Nebula This nebula, found in the constellation Taurus, is the remains of a supernova that took placein 1054.

A supernova event is thought to be triggered when a massive star consumes most of its nuclear fuel. Without a heatengine to generate the gas pressure required to balance its immense gravitational field, the star collapses. Thisimplosion, or bursting inward, is huge, resulting in a shock wave that moves out from the star’s interior. Thisenergetic shock wave destroys the star and blasts the outer shell into space, generating the supernova event.

H-R Diagrams and Stellar EvolutionHertzsprung-Russell diagrams have been helpful in formulating and testing models of stellar evolution. They arealso useful for illustrating the changes that take place in an individual star during its life span. Refer back to Figure10, which shows the evolution of a star about the size of the sun. Keep in mind that the star does not physicallymove along this path. Its position on the H-R diagram represents the color and absolute magnitude of the star atvarious stages in its evolution.

Stellar RemnantsEventually, all stars consume their nuclear fuel and collapse into one of three documented states—white dwarf,neutron star, or black hole. Although different in some ways, these small, compact objects are all composed ofincomprehensibly dense material and all have extreme surface gravity.

White DwarfsWhite dwarfs are the remains of low-mass and medium-mass stars. They are extremely small stars with densitiesgreater than any known material on Earth. Although some white dwarfs are no larger than Earth, the mass of such adwarf can equal 1.4 times that of the sun. So, their densities may be a million times greater than water. A spoonfulof such matter would weigh several tons. Densities this great are possible only when electrons are displaced inward

from their regular orbits, around an atom’s nucleus, allowing the atoms to take up less than the “normal” amount ofspace. Material in this state is called degenerate matter.In degenerate matter, the atoms have been squeezed together so tightly that the electrons are displaced much nearerto the nucleus. Degenerate matter uses electrical repulsion instead of molecular motion to support itself from totalcollapse. Although atomic particles in degenerate matter are much closer together than in normal Earth matter, theystill are not packed as tightly as possible. Stars made of matter that has an even greater density are thought to exist.As a star contracts into a white dwarf, its surface becomes very hot, sometimes exceeding 25,000 K. Even so,without a source of energy, it can only become cooler and dimmer. Although none have been observed, the laststage of a white dwarf must be a small, cold body called a black dwarf. Table 2 summarizes the evolution of stars ofvarious masses. As you can see, the sun begins as a nebula, spends much of its life as a main-sequence star,becomes a red giant, planetary nebula, white dwarf, and finally, black dwarf.

Neutron StarsAfter studying white dwarfs, scientists made what might at first appear to be a surprising conclusion. The smallestwhite dwarfs are the most massive, and the largest are the least massive. The explanation for this is that a moremassive star, because of its greater gravitational force, is able to squeeze itself into a smaller, more densely packedobject than can a less massive star. So, the smaller white dwarfs were produced from the collapse of larger, moremassive stars than were the larger white dwarfs.This conclusion led to the prediction that stars smaller and more massive than white dwarfs must exist. Theseobjects, called neutron stars, are thought to be the remnants of supernova events. In a white dwarf, the electrons arepushed close to the nucleus, while in a neutron star, the electrons are forced to combine with protons to produceneutrons. If Earth were to collapse to the density of a neutron star, it would have a diameter equal to the length of afootball field. A pea-size sample of this matter would weigh 100 million tons. This is approximately the density ofan atomic nucleus. Neutron stars can be thought of as large atomic nuclei.

SupernovaeDuring a supernova, the outer layer of the star is ejected, while the core collapses into a very hot neutron star about20 kilometers in diameter. Although neutron stars have high surface temperatures, their small size would greatlylimit their brightness. Finding one with a telescope would be extremely difficult.

Figure 14 Veil Nebula Located in the constellation Cygnus, this nebula is the remnant of an ancientsupernova.

However, astronomers think that a neutron star would have a very strong magnetic field. Further, as a star collapses,it will rotate faster, for the same reason ice skaters rotate faster as they pull in their arms. Radio waves generated bythese rotating stars would be concentrated into two narrow zones that would align with the star’s magnetic poles.Consequently, these stars would resemble a rapidly rotating beacon emitting strong radio waves. If Earth happenedto be in the path of these beacons, the star would appear to blink on and off, or pulsate, as the waves swept past.In the early 1970s, a source that radiates short bursts or pulses of radio energy, called a pulsar, was discovered inthe Crab Nebula. Studies of this radio source revealed it to be a small spinning star centered in the nebula. Thepulsar found in the Crab Nebula is undoubtedly the remains of the supernova of 1054.

Black HolesAre neutron stars made of the most dense materials possible? No. During a supernova event, remnants of stars threetimes more massive than the sun apparently collapse into objects even smaller and denser than neutron stars. Eventhough these objects, called black holes, are very hot, their gravity is so strong that not even light can escape theirsurface. So they disappear from sight. Anything that moves too near a black hole would be swept in by its gravityand lost forever.How can astronomers find an object whose gravitational field prevents the escape of all matter and energy? Onestrategy is to find evidence of matter being rapidly swept into a region of apparent nothingness. Scientists think thatas matter is pulled into a black hole, it should become very hot and emit a flood of X-rays before being pulled in.Because isolated black holes would not have a source of matter to swallow up, astronomers first looked at binary-star systems.A likely candidate for a black hole is Cygnus X-1, a strong X-ray source in the constellation Cygnus. In this case,the X-ray source can be observed orbiting a supergiant companion with a period of 5.6 days. It appears that gasesare pulled from this companion and spiral into the disk-shaped structure around the black hole, as shown in Figure15.

Figure 15 Black Hole Gases from the red giant spiral into the black hole.

Section 3 The Universe

Key Concepts What is the size and structure of the Milky Way Galaxy? In what ways do galaxies differ from one another? What evidence indicates that the universe is expanding? According to the big bang theory, how did the universe begin?

Vocabulary galaxy galaxy cluster Hubble’s law big bang theory

On a clear and moonless night away from city lights, you can see a truly marvelous sight—our own Milky WayGalaxy, as shown in Figure 16. Galaxies are groups of stars, dust, and gases held together by gravity. There may bemore than 100 billion stars in the Milky Way Galaxy alone. Our galaxy looks milky because the solar system islocated within a flat disk—the galactic disk. We view it from the inside and see stars in every direction.

Figure 16 Milky Way Galaxy Notice the dark band caused by interstellar dark nebulae.

The Milky Way GalaxyWhen astronomers began to survey the stars located along the plane of the Milky Way, it appeared that equalnumbers lay in every direction. Could Earth actually be at the center of the galaxy? Scientists came up with a betterexplanation. Imagine that the trees in an enormous forest represent the stars in the galaxy. After hiking into thisforest, you look around. You see an equal number of trees in every direction. Are you in the center of the forest?Not necessarily. Anywhere in the forest will seem to be the center, except at the very edge.

Size of the Milky WayIt’s hard to study the Milky Way Galaxy with optical telescopes because large quantities of interstellar matter blockour vision. With the aid of radio telescopes, scientists have determined the structure of our galaxy. The Milky Wayis a large spiral galaxy whose disk is about 100,000 light-years wide and about 10,000 light-years thick at thenucleus, as shown in Figure 17A. As viewed from Earth, the center of the galaxy lies beyond the constellationSagittarius. Figure 17B shows an edge-on view of the Milky Way.

Figure 17 Structure of the Milky Way A The spiral arms are clearly visible in the overhead view of our galaxy. BOur solar system is located about 30,000 light-years from the galactic nucleus.

Structure of the Milky WayRadio telescopes reveal that the Milky Way has at least three distinct spiral arms, with some signs of splintering.The sun is positioned in one of these arms about two thirds of the way from the center, or galactic nucleus, at adistance of about 30,000 light-years. The stars in the arms of the Milky Way rotate around the galactic nucleus. Themost outward arms move the slowest, and the ends of the arms appear to trail. Our solar system orbits the galacticnucleus about every 200 million years.Surrounding the galactic disk is a nearly round halo made of thin gas and numerous clusters of stars. These starclusters do not participate in the rotating motion of the arms but have their own orbits that carry them through thedisk. Although some clusters are very dense, they pass among the stars of the arms with plenty of room to spare.

Types of GalaxiesIn the mid-1700s, German philosopher Immanuel Kant proposed that the fuzzy patches of light scattered among thestars were actually distant galaxies like the Milky Way. Today we know that the universe includes hundreds ofbillions of galaxies, each containing hundreds of billions of stars. From these hundreds of billions of galaxies,scientists have identified several basic types.

Spiral GalaxiesAs shown in Figure 18A, spiral galaxies are usually disk-shaped, with a somewhat greater concentration of starsnear their centers. There are numerous variations, though. Viewed broadside, the arms are often seen extendingfrom the central nucleus and sweeping gracefully away. The outermost stars of these arms rotate most slowly,giving the galaxy the appearance of a pinwheel.

Figure 18 Spiral Galaxies A A spiral galaxy looks somewhat like a pinwheel. B A barred spiral galaxy has a barthrough its center, with arms extending outward from the bar.

One type of spiral galaxy, however, has its stars arranged in the shape of a bar, which rotates as a rigid system.Attached to each end of these bars are curved spiral arms. These have become known as barred spiral galaxies, asshown in Figure 18B. Recent evidence indicates that the Milky Way may be a barred spiral galaxy. Spiral galaxiesare generally quite large. About 10 percent of all galaxies are thought to be barred spirals, and another 20 percentare regular spiral galaxies.

Elliptical GalaxiesAbout 60 percent of galaxies are classified as elliptical galaxies. Elliptical galaxies range in shape from round tooval. Although most are small, the very largest known galaxies—200,000 light-years in diameter—are elliptical.This type of galaxy, shown in Figure 19, does not have spiral arms.

Figure 19 Elliptical Galaxy Most galaxies are classified as elliptical with shapes ranging from round to oval.

Irregular GalaxiesOnly 10 percent of the known galaxies have irregular shapes and are classified as irregular galaxies. The best-known irregular galaxies, the Large and Small Magellanic Clouds, are easily visible with the unaided eye. Thesegalaxies were named after the explorer Ferdinand Magellan, who observed them when he sailed around Earth in1520. They are our nearest galactic neighbors—only 150,000 light-years away. An irregular galaxy is shown inFigure 20.

Figure 20 Irregular Galaxy Irregular galaxies have irregular shapes. Describing What type of stars would you findin an irregular galaxy?

In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars.Irregular galaxies are composed mostly of young stars, while elliptical galaxies contain old stars. The Milky Wayand other spiral galaxies have both young and old stars, with the youngest located in the arms.

Galaxy ClustersOnce astronomers discovered that stars were found in groups, they wondered whether galaxies also were groupedor just randomly distributed throughout the universe. They found that, like stars, galaxies are grouped in clusters.One such cluster is shown in Figure 21. Some clusters may contain thousands of galaxies. Our own cluster, calledthe Local Group, contains at least 28 galaxies. Of these, three are spirals, 11 are irregulars, and 14 are ellipticals.Galaxy clusters also make up huge groups called superclusters. Studies indicate that superclusters may be thelargest entities in the universe.

Figure 21 Galaxy Cluster This cluster of galaxies is located about 1 million light-years from Earth.

The Expanding UniverseRecall the Doppler effect that you read about in Chapter 24. Remember that when a source is moving away, its lightappears redder than it actually is, because its waves appear lengthened. Objects approaching have their light wavesshifted toward the blue or shorter wavelengths. Therefore, the Doppler effect reveals whether a star or other body inspace is moving away from Earth or toward Earth. The amount of shift allows us to calculate the rate at which therelative movement is occurring. Large Doppler shifts indicate higher speeds; smaller Doppler shifts indicate lowerspeeds.

Red ShiftsOne of the most important discoveries of modern astronomy was made in 1929 by Edwin Hubble. Observationscompleted several years earlier revealed that most galaxies have Doppler shifts toward the red end of the spectrum.The red shift occurs because the light waves are “stretched,” which shows that Earth and the source are movingaway from each other. Hubble set out to explain this red shift phenomenon.Hubble realized that dimmer galaxies were probably farther away than were brighter galaxies. He tried to determinewhether a relationship existed between the distances to galaxies and their red shifts. Hubble used estimated

distances based on relative brightness and Doppler red shifts to discover that galaxies that exhibit the greatest redshifts are the most distant.

Hubble’s LawA consequence of the universal red shift is that it predicts that most galaxies—except for a few nearby—are movingaway from us. Recall that the amount of Doppler red shift depends on the speed at which the object is movingaway. Greater red shifts indicate faster speeds. Because more distant galaxies have greater red shifts, Hubbleconcluded that they must be retreating from us at greater speeds. This idea is currently termed Hubble’s law. Itstates that galaxies are retreating from us at a speed that is proportional to their distance.Hubble was surprised at this discovery because it implied that the most distant galaxies are moving away from usmany times faster than those nearby. What does this mean? The red shifts of distant galaxies indicate that theuniverse is expanding.To help visualize the nature of this expanding universe, imagine a loaf of raisin bread dough that has been set out torise for a few hours. As shown in Figure 22, as the dough doubles in size, so does the distance between all of theraisins. However, the raisins that were originally farther apart traveled a greater distance in the same time span thanthose located closer together. We therefore conclude that in an expanding universe, as in the raisin bread doughanalogy, those objects located farther apart move away from each other more rapidly.

Figure 22 Raisin Dough Analogy As the dough rises, raisins that were farther apart travel a greater distance in thesame time as those that were closer together. Like galaxies in an expanding universe, the distant raisins move awayfrom one another more rapidly than those that are near one another.Another feature of the expanding universe can be demonstrated. No matter which raisin you select, it will moveaway from all the other raisins. Likewise, no matter where one is located in the universe, every other galaxy—again, except those in the same cluster—will be moving away. Hubble had indeed advanced our understanding ofthe universe. The Hubble Space Telescope is named in his honor.

The Big BangDid the universe have a beginning? Will it have an end? Scientists are trying to answer these questions.Any theory about the origin of the universe must account for the fact that all distant galaxies are moving away fromus. Because all galaxies appear to be moving away from Earth, is our planet in the center of the universe? Probablynot, because if we are not even in the center of our own solar system, and our solar system is not even in the centerof the galaxy, it seems unlikely that we could be in the center of the universe.A more probable explanation exists. Imagine a balloon with paper-punch dots glued to its surface. When theballoon is inflated, each dot spreads apart from every other dot. Similarly, if the universe is expanding, every galaxywould be moving away from every other galaxy.This concept of an expanding universe led to the widely accepted big bang theory. According to the big bangtheory, the universe began as a violent explosion from which the universe continues to expand, evolve, and cool.The big bang theory states that at one time, the entire universe was confined to a dense, hot, supermassive ball.Then, about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions. The bigbang, as shown in Figure 23, marks the beginning of the universe. All matter and space were created at that instant.After several hundred thousand years, the universe became cool enough for atoms to form. Gases in the universecontinued to cool and condense. They eventually formed the stars that make up the galaxies we now observemoving away from us.

Figure 23 The Big Bang According to the big bang theory, the universe began 13.7 billion years ago. Twohundred million years later, the first stars and galaxies began to form.

Supporting EvidenceThrough decades of experimentation and observation, scientists have gathered substantial evidence that supports thebig bang theory. For example, the red shift of galaxies that you read about earlier indicates that the universe is stillexpanding. Scientists discovered a type of energy called cosmic background radiation. This energy was detected asfaint radio signals coming from every direction in space. Scientists think that this radiation was produced during thebig bang.

The Big Crunch?If the universe began with a big bang, how will it end? One view is that the universe will last forever. In thisscenario, the stars will slowly burn out, being replaced by invisible degenerate matter and black holes that willtravel outward through an endless, dark, cold universe. The other possibility is that the outward flight of thegalaxies will slow and eventually stop. Gravitational contraction would follow, causing the galaxies to collide andcombine into the high-energy, high-density mass from which the universe began. This fiery death of the universe,the big bang operating in reverse, has been called the “big crunch.”Whether or not the universe will expand forever or eventually collapse upon itself depends on its average density. Ifthe average density of the universe is more than its critical density—about one atom for every cubic meter—thegravitational field is enough to stop the outward expansion and cause the universe to contract. On the other hand, ifthe density of the universe is less than the critical value, it will expand forever. Current estimates of the density ofthe universe place it below the critical density, which predicts an ever-expanding, or open, universe. Additionalsupport for an open universe comes from studies that indicate the universe is expanding faster now than in the past.The view currently favored by most scientists is an expanding universe with no ending point.It should be noted, however, that the methods used to determine the ultimate fate of the universe have substantialuncertainties. It is possible that previously undetected matter exists in great quantities in the universe. If this is so,the galaxies could, in fact, collapse in the “big crunch.”